Descriptions

Erythrocytic inclusion body syndrome (EIBS), a viral disease
of salmon, was investigated. Reliable artificial infections were
established in coho salmon (Oncorhynchus kisutch) via
intraperitoneal injection of homogenized kidney, spleen, and blood
from naturally infected animals. By examining the exposed fish
periodically for viral inclusion bodies and hematocrits, the disease
progression for EIBS was determined. Erythrocytic inclusion body
syndrome had four stages which were incubation, inclusion body
formation, lysis, and recovery. After an incubation period,
circulating erythrocytes developed cytoplasmic inclusions and the
numbers of infected cells increased to a maximum. The lysis stage
followed and was characterized by lysis of erythrocytes with
inclusions, lower hematocrits, and an accompanying anemia. When
hematocrits were lowest, no cells with inclusions were observed.
Following the anemia, the fish began to recover as indicated by an
increase in circulating lymphocytes and immature erythrocytes.
When the fish had recovered, their hematocrits were normal,
serum was protective in passive immunization experiments, the
fish showed resistance to reinfection, and tissues from the
recovered fish were no longer infectious. The four stages of EIBS
were discerned in infected fish held at temperatures of 6, 9, 12, 15,
and 18 C but as the temperature increased, the incubation time,
time to maximum inclusions, duration of the inclusion stage, and
the time to recovery decreased. Numbers of inclusions were
highest in fish held at 6 and 9 C. In histological analysis, cells of
the liver, anterior kidney, and spleen from infected fish were not
necrotic. Hemosiderin, a protein-iron pigment accumulated in the
spleen following cell lysis. Utilizing the artificial infection, rainbow
(Oncorhynchus mykiss) and cutthroat (Oncorhynchus clarki) trout
were determined to be susceptible, but the disease was much less
severe in these species than in chinook (Oncorhynchus
tshawytscha) and coho salmon, the natural hosts of the virus.
Additional results indicated EIBS is transmitted horizontally and
that infected fish may be immunosuppressed. Staining of blood
smears with acridine orange has indicated the genome of the virus
is single stranded nucleic acid.